Power steering

ABSTRACT

In an electric power assisted steering system a power stage ( 2 ) is provided comprising at least a first conducting means ( 4 ), and a control stage ( 1 ) is provided comprising a second separate conducting means ( 5 ) adapted to carry one or more electrical components of a control circuit. Each conducting means comprising one or more conductive tracks or links ( 10 ) for carrying current to and/or from components of the electric power assisted steering system.

This invention relates to improvements in electric power assistedsteering systems, and in particular to improvements in the arrangementof electrical circuitry in such systems.

Electric power assisted steering systems are well known in the art. Atypical system comprises a first portion of shaft operatively connectedto a steering wheel and a second portion of shaft operatively connectedto a road wheel of the vehicle through, for example, a steering rack. Atorque sensor provides an indication of the torque in the first portionof shaft, and produces an output signal dependent on that torque whichis passed to an electrical circuit. The electrical circuit processes theoutput signal to produce a motor drive signal for a motor which isconnected through a gearbox to the second portion of shaft. The motordrive signal corresponds to an assistance torque to be applied by themotor to the second portion of shaft to assist the driver of thevehicle, thus making the steering wheel easier to turn.

The electrical circuit comprises low power devices which modify theoutput signal from the torque sensor to produce the motor drive signal.This may typically comprise an electronic control unit comprising anintegrated circuit or discrete components. The motor drive signal isthen further processed by the circuit to produce one or more motorvoltage current signals indicate of the current required in each phaseof the electric motor. For a typical three phase motor, a pulse widthmodulation strategy can be employed to drive the motor, whereby themotor drive signal is converted into three motor phase voltage signalswhich control suitable high power switching circuitry to apply theappropriate voltage waveform to each motor phase. A capacitor may alsobe provided which is placed in the motor current path to enable ameasurement of motor current to be made. The electronic and electricalcircuitry can therefore be considered to be loosely divided into twostages. A first (control) stage comprises the low power components usedto modify the torque sensor output signal and generate the appropriatemotor drive signal (s). The second stage (power stage) comprises thehigh power electrical components and associated electrical conductors ofthe electrical switching stage, typically a bridge circuit of high powersemiconductor switching devices, for example transistors, and the motorcurrent sense resistor.

It is known in the art in electric power assisted steering systems ofthe kind set forth to provide both the control circuit and the powerstage on a single printed circuit board.

We are also aware of U.S. Pat. No. 5,602,451 in which an electric powerassisted steering system is disclosed having both low power componentsand high power components located on separate printed circuit boards.

In accordance with a first aspect, the invention provides an electricpower assisted steering system of the kind which comprises a firstportion adapted to be operatively connected to a steering wheel of avehicle, a second portion adapted to be operatively connected to a roadwheel, a torque sensor adapted to provide an output signal indicative ofthe torque in the first portion, and an electrical circuit adapted toprocess the output signal from the torque sensor to produce a drivesignal for a motor connected through a gearbox to the second portion. inwhich the electrical circuit comprises a power stage which comprises atleast a first conducting means and a control stage comprising a secondseparate conducting means adapted to carry one or more electricalcomponents of a control circuit, each conducting means comprising one ormore conductive tracks or links for carrying current to and/or fromcomponents of the electric power assisted steering system, and in whichan electromagnetic shield is provided between the first and secondconducting means.

Preferably, the first and second conducting means are arranged spacedapart one on top of the other, for example with an air gap therebetween.

By providing two separate stages, each comprising a separate conductingmeans, it is possible to reduce the cost of the control stage comparedwith a prior art system in which both the power and control componentsare provided on one conducting means as the power handling capability ofthe conducting tracks or links between the components of the controlstage can be made lower than that of the tracks or links betweencomponents of the power stage.

By conducting means we mean a printed circuit board, lead frame or anyother structure comprising a number of conducting tracks or connectionsadapted to link together one or more of the components of the respectivestage to form a circuit and supported or joined by a non-electricallyconducting material.

In a most preferred arrangement, the conducting track or connections ofthe power stage conducting means have a higher power rating than thoseof the control stage carrier means.

For example, the power stage may comprise a number of conducting tracksencapsulated in a non-conducting plastic material, the tracks beingadapted to interconnect at least some of the power stage components. Theconducting tracks may be provided in the form of multiple layers ofconducting material, such as copper tracks. The power rating isincreased by increasing the number of layers for each track or thecross-sectional area of each track.

It is most preferred that the conducting tracks and plastics materialare selected such that the power stage forms a substantially rigidplanar lead frame. The tracks may then be in the form of a fretwork. Thelead frame may include more than one piece of fretwork. Each piece ortrack may be of different metals to suit different soldering or otherconnecting techniques. The leadframe does not need to support any of thepower stage components although it could be adapted to support one ormore components, such as a current capacitor.

A portion of one or more of the conducting tracks may be upstanding fromthe planar frame for interconnecting with tracks or components of thecontrol stage.

In a most preferred arrangement, the first conducting means comprises asingle layer fretwork of conducting tracks embedded in a plasticsmaterial. The fretwork may be stamped from a sheet of material such ascopper, and placed in a mould so that the plastic material can beinjection moulded around it. This produces a rigid carrier means ontowhich various components can be mounted. The power capacity can then beeasily changed from one device to another by increasing thecross-sectional area of the fretwork whilst retaining the same mould.

The control stage preferably comprises a printed circuit board definingthe second conducting means upon which the control stage components aremounted. Some or all of these low power components may be surfacemounted to minimise both.

The printed circuit board most preferably comprises a four layer 1 _(oz)circuit board. Of course, more or less layers, and different weights oftracks could be employed within the scope of the invention.

Preferably, an electromagnetic shield is provided between the first andsecond conducting means. The shield may comprise a conducting screenhaving holes or other openings through which the upstanding conductorsof the power stage can pass to contact the tracks or components in thecontrol stage, or vice versa. The electromagnetic shield provides abarrier to substanitially prevent electromagnetic radiation from onestage interfering with components of the other stage. This is preferablyearthed by a low impedance conducting connection or link to a suitableearth point.

One or more of the high power devices comprising part of the power stagemay be provided on a separate substrate and electrically connected tothe conductors of the first conducting means. This may be by solderjoints to portions of the tracks either upstanding or depending from thefirst conducting means.

The separate substrate may comprise a material having good thermalconducting and electrical isolation properties, such as IMS. Thesubstrate may be directly connected to a portion of a housing of theelectrical motor to dissipate heat. Alternatively, it may be directlyconnected to a portion of the gearbox which is provided between themotor and the first portion of shaft. The motor housing and/or gearboxtherefore dissipates heat generated in the power devices without theneed for a separate heatsink. Or course, a separate heatsink could beprovided as well/or instead if required.

By providing the first conducting means which provides an electricalconnection between components of the power stage and a second conducingmeans (carrying the control stage) one above the other with anelectromagnetic shield between them and securing them directing onto themotor or gearbox, a compact assembly is produced which can be maderelatively immune to external electromagnetic interference if the shieldis directly connected to the motor or gearbox.

The electromagnetic compatibility of the assembly can be furtherimproved by providing fixing means that co-operates with the shield toprovide a mechanical connection from the control stage to the motorhousing which is earthed.

Preferably, the power stage conducting means is positioned relative themotor so as to minimise the length of the conductive tracks between themotor windings and the power components such as the motor drive stageswitching devices. This ensures optimum efficiency and a minimum powerdissipation due to impedance of the conducting tracks or links of thecarrier means.

In a most preferred arrangement, the power stage includes a motorcurrent sense resistor which is located substantially in the plane ofthe conducting fretwork of the first conducting means. In this manner,there are no power conducting paths above or below the high powerresistor so mutually induced inductance is minimised.

Since the motor current sense resistor is a relatively large component,placing it in the same plane as the fretwork reduces the height of theassembly. Of course, this can also be achieved when the first conductingmeans comprises a multi-layer circuit board by placing the resistor inthe plane of the circuit board.

By being “in the plane”, we mean that the axial centre of the resistoris substantially in line with a plane passing through the centre of thecarrier means. However, any arrangement whereby at least part of theresistor is above and part of the resistor is below a surface of thefirst conducting means is envisaged.

A magnetic shield material may be provided on the first conducting meanswhich encapsulates the sense resistor on both sides, or perhaps just oneside to reduce electromagnetic interference.

The first conducting means may also carry a connector means whichenables electrical connection to be made to one or more the conductingtracks or components for testing or remote operation purposes.

There will now be described by way of example only, one embodiment ofthe present invention with reference to the accompanying drawings ofwhich:

FIG. 1 is an illustration of the arrangement of the power stage, controlstage and a motor in a electrical power assisted steering system inaccordance with the present invention;

FIG. 2 is an enlarged view showing an alternative arrangement;

FIG. 3 shows a metallic bush being moulded into the frame; and

FIG. 4 shows the location of the circuitry relative to the motor.

FIG. 1 shows a part of an electrical power assisted steering systemcomprising a control stage 1 adapted to produce one or more motor drivesignals and a power stage 2 adapted to control current flowing in thewindings of an electric motor (not shown) in response to the motor drivesignals. The motor drive signal is typically processed by a controlcircuit to generate three motor phase voltage or current signals, onefor each phase of a three phase brushless DC motor. The power circuitcomprises a three phase bridge of power switching devices which areswitched in response to the three motor phase signals. For high torqueelectric motors, the switching devices must handle high currents anddissipate high power.

As shown, at least some of the components 3 of the power circuitry 2 areprovided on a first conducting means 4 and the control circuitry isprovided on a second, separate, conducting means 5 positioned above thefirst carrier means 4 and separated therefrom by an air gap 6.

The first conducting means 4 comprises a conducting fretwork 10 ofrelatively thick track, for example stamped from a sheet of coppermaterial. The fretwork is then placed in a mould and encapsulated (atleast partially) by plastics 11 to form a substantially rigid assemblyor leadframe, and with a vibration clip 12 forming part of the plastics11 encapsulating the fretwork of the leadframe. Power components such asa EMC capacitor 3 are provided on the lead frame in electrical contactwith the fretwork 10.

Other power devices 23 of the power circuit 2 are provided on a separatesubstrate 20. This comprises a high thermal conductivity, electricallyinsulating substrate which is mounted directly onto a thermallyconducting portion of the motor housing 22. Electrical connectionbetween the fretwork 10 of the leadframe and the power devices 23 ismade by soldering the power devices 23 onto portions 24 of the fretworkdepending from the plane of the lead frame.

The control stage 1 is arranged above and spaced from the power stage 2and comprises a multi-layer printed circuit board 5 upon which thecontrol circuit components 30 are mounted. Electrical contact betweenthe control stage 1 and the power stage 2 is provided by connection withportions 31 of the fretwork 10 which are upstanding from the leadframe.An electromagnetic shield 40 comprising a conducting screen or mesh isprovided in the air gap 6 between the leadframe and the printed circuitboard to screen the control stage 1 from the power stage 2. Theelectrical contacts between the two stages pass through holes in theshield. The shield 40 can be connected to the motor casing earthed tochassis.

By making the fretwork 10 from relatively thick conducting material, theleadframe can be made self supporting and, as shown, the control stage 1can be entirely supported over the power stage 2 by the upstandingportions 31 of fretwork 10 which define the electrical connectionsbetween the two stages.

An alternative arrangement is shown in FIG. 2, which represents anenlarged view of a portion of the control stage 1 positioned above thepower stage 2. As shown, a raised portion 50 of the chassis (i.e. motorhousing) passes through the power stage lead frame to support thecontrol stage directly. A portion of the shield 40 is clamped betweenthe control stage and the raised portion to provide a good electricalcontact by, for example, a screw or bolt in a bore raised portion 50.

As shown in FIG. 3 of the accompanying drawings a metallic insert bush55 is moulded into the frame 10 and both of its ends protrude from theframe. A screw 51 clamps the board and shield of the chassis in a mannerto provide a good electrical contact.

It is preferred that the motor housing 22 is made as a metal castingchassis. The metal helps to dissipate heat from the power devices. In afurther refinement, as shown in FIGS. 4(a) and (b) the motor housing 22is connected to a separate gearbox housing 60 which is also of metal.The connections from the motor windings (not shown) to the power stageare arranged close to the interface between the motor housing 22 andgearbox housing 60 so that the power devices can be located on thesubstrate connected to the motor housing close to the gearbox housing.At the same time the length of the connection to the windings isminimised. As can be seen from the cut-away view in FIG. 3(b), the powerstage and control stage are enclosed within a housing 70 to provideimmunity from external electromagnetic radiation. A connector 71 whichcontacts the electrical circuitry is provided in a face or the lid ofthe housing 70 but typically the end face as shown to enable a powersupply to be connected to the circuit and feeds from other sensors orcircuitry. This can be used for diagnostics. The power device and thesubstrate are bolted or soldered directly to a raised portion 72 of themotor housing 22 directly above the motor phase windings (not shown).

What is claimed is:
 1. An electric power assisted steering system for avehicle having a steering wheel and a road wheel, wherein the systemcomprises a motor, a gearbox, a first portion adapted to be operativelyconnected to said steering wheel, a second portion adapted to beoperatively connected to said road wheel, a torque sensor adapted toprovide an output signal indicative of the torque in said first portion,an electrical circuit adapted to process said output signal from saidtorque sensor to produce a drive signal for said motor connected throughsaid gearbox to said second portion, and a control circuit including atleast one electric component, wherein said electrical circuit comprisesa power stage which comprises at least a first conducting means and acontrol stage comprising a second separate conducting means adapted tocarry said electrical component of said control circuit, each saidconducting means comprising at least one conductive track for carryingcurrent to and/or from components of said electric power assistedsteering system, and wherein an electromagnetic shield is providedbetween said first and second conducting means.
 2. An electric powerassisted steering system according to claim 1, wherein said first andsecond conducting means are arranged spaced apart one on top of theother.
 3. An electric power assisted steering system according to claim1, wherein said conducting track or connections of said power stageconducting means have a higher power rating than those of said controlstage carrier means.
 4. An electric power assisted steering systemaccording to claim 1, wherein said power stage comprises a number ofconducting tracks encapsulated in a non-conducting plastic material,said track being adapted to interconnect at least some of said powerstage components.
 5. An electric power assisted steering systemaccording to claim 4, wherein said conducting tracks of said power stagecomprise multiple layers of conducting material, such as copper tracks.6. An electric power assisted steering system according to claim 4,wherein said power stage forms a substantially rigid planar lead framewith said tracks in the form of a fretwork.
 7. An electric powerassisted steering system according to claim 6, wherein said power stageincludes a motor current sense resistor which is located substantiallyin the plane of said conducting fretwork of said first conducting means.8. An electric power assisted steering system according to claim 7,wherein a portion of said conducting track of said power stage isupstanding from said planar frame for interconnecting with tracks ofsaid control stage.
 9. An electric power assisted steering systemaccording to claim 1, wherein said control stage comprises a printedcircuit board defining said second conducting means upon which saidcontrol stage components are mounted.
 10. An electric power assistedsteering system according to claim 1, wherein at least one high powerdevice comprising part of said power stage is provided on a separatesubstrate and electrically connected to said conductors of said firstconducting means.